CamelSession: Simplify trust_prompt() signature.

Let the CamelSession subclass encode the certificate data if it needs to.
That's an Evolution-specific implementation detail Camel doesn't need to
know about.  The method signature is now:

  CamelCertTrust  (*trust_prompt)  (CamelSession *session,
                                    CamelService *service,
                                    GTlsCertificate *certificate,
                                    GTlsCertificateFlags errors);